1. Field of the Invention
The invention relates to a field emission electron source and a method of fabricating the same, and more particularly to an improvement in a structure of a bonding pad to be used for wire-bonding.
2. Description of the Related Art
By applying an electric field having an intensity of 1.times.10.sup.9 [v/m] to a surface of metal or semiconductor, electrons pass over a barrier by virtue of the tunnel effect, and thus it is possible to carry out electron emission in vacuum even at room temperature. This phenomenon is called "field electron emission". A cathode emitting electrons under the above mentioned principle is called a "field emission cathode".
These days, it has become possible to fabricate a micron-sized cathode by employing semiconductor integration technique. As an example of such a micro-sized cathode is known a field emission cathode called "Spindt" type cathode. By employing technique for fabricating a micron-sized semiconductor device, it is possible in the "Spindt" type field emission cathode to establish a sub-micron sized gap between a conically shaped emitter and a gate electrode which attracts electrons thereto. Thus, by applying a voltage in two digits across an emitter and a gate electrode, the emitter emits electrons to thereby generate a current ranging from 0.1 .mu.A to 10.mu.m per an emitter. In addition, since it is possible to fabricate the cathode with a pitch between adjacent emitters being in the range of a few microns to tens of microns, hundreds of thousand emitters can be arranged on a substrate. Hence, it is possible to obtain a high output current, specifically in the range of a few mA to about 100 mA, at a low voltage, and thus the "Spindt" type field emission cathode is now expected to be used as an electron source for a cathode ray tube (CRT), an electron microscope (EM) and a flat panel display.
The above mentioned field emission cathode may be made of electrically conductive silicon (Si), tungsten (W) or tantalum (Ta). However, the cathode is mostly made of molybdenum (Mo), because Mo can be readily processed and has a relatively small work function. FIGS. 1A to 1F are cross-sectional views showing respective step in a method of fabricating the "Spindt" type field emission electron source employing a molybdenum cathode.
First, as illustrated in FIG. 1A, insulating films 52 such as a thermally oxidized film are formed on upper and lower surfaces of a substrate 51 which is either an electrically conductive substrate or an electrically insulating substrate formed at a surface thereof with an electrically conductive layer. Then, a tungsten silicide film 53, which will make a gate electrode, wire and a bonding pad, is formed on one of the insulating films 52.
Then, as illustrated in FIG. 1B, photoresist 54 is applied over the tungsten silicide film 53 and subsequently patterned by employing generally used semiconductor photolithography technique. Then, the photoresist 54 in an area other than an area where a gate electrode, wire and a bonding pad is to be formed is removed by anisotropic dry, reactive ion etching (RIE).
Then, as illustrated in FIG. 1C, photoresist 55 is applied over the tungsten silicide film 53 and patterned by employing semiconductor photolithography technique. Subsequently, the tungsten silicide film 53 and the insulating film 42 are removed in an area where emitters are to be formed, to thereby form gate holes 56. Since a diameter of the gate holes 56 is in the sub-micron order and a space between the adjacent gate holes 56 is small, specifically in the range of a few microns to tens of microns, the formation of the gate holes 56 is carried out by anisotropic dry, reactive ion etching (RIE).
Then, the rest of the photoresist 55 is removed. Then, as illustrated in FIG. 1D, a sacrifice layer 57 made of aluminum or alumina is formed on the tungsten silicide film 53 by evaporation. The reason of the formation of the sacrifice layer 57 is to make it easy to remove unnecessary portions a of molybdenum layer deposited in a later mentioned step. The aluminum or alumina sacrifice layer 57 is formed so that the sacrifice layer 57 is formed on sidewalls of the tungsten silicide film 53 and the insulating layer 52 both of which are exposed to the gate holes 56, but is not formed on an exposed surface 60 of the substrate 51 which surface defines a bottom of the gate holes 56.
Then, as illustrated in FIG. 1E, a molybdenum layer 61 is formed over the sacrifice layer 57 by evaporation to thereby form emitters 62. In evaporation of molybdenum, as molybdenum is gradually evaporated on sidewalls of the gate holes 56, a diameter of the gate holes 56 gradually decreases. Thus, there are formed conically shaped emitters 62 having a sharpened apex.
Then, the substrate 51 is soaked into heated phosphoric acid solution to thereby remove the sacrifice layer 57. The molybdenum layer 61 deposited on the aluminum or alumina sacrifice layer 57 are removed together with the sacrifice layer 57, and hence, as illustrated in FIG. 1F, there are left only the patterned insulating film 52 and tungsten silicide film 53, and the molybdenum emitters 62 on the substrate 51.
FIG. 2 is a perspective view illustrating a conventional field emission electron source fabricated in accordance with the method having been explained with reference to FIGS. 1A to 1F. The illustrated field emission electron source includes a substrate 51 which is either an electrically conductive substrate or an electrically insulating substrate formed at a surface thereof with an electrically conductive layer, one or more emitters 62 made of molybdenum and having a sharpened apex, a gate electrode 75 formed with gate holes 56 surrounding the apexes of the emitters 62, a bonding pad 66 from which a voltage can be applied to the gate electrode 75 through a wire 68 (Not illustrated in FIG. 2. See FIG. 3.), a wire 65 for electrically connecting the gate electrode 75 to the bonding pad 66, and an insulating film 52 formed on the substrate 51. The gate electrode 75, the wire 65 and the bonding pad 66 are formed integrally with one another to thereby constitute a tungsten silicide layer 53. The tungsten silicide layer 53 is electrically insulated from the substrate 51 by the insulating film 52. The above mentioned elements 52, 53, 56 and 62 are all formed on the substrate 51.
FIG. 3 is a cross-sectional view of an example of how the above mentioned conventional field emission electron source is mounted on a package. A package 67 is formed at a surface thereof with an electrically conductive layer 71 on which a field emission electron source is to be mounted. The field emission electron source is fixedly secured to the package 67 on the electrically conductive layer 71 through an adhesive layer 72 made of electrically conductive adhesive or Au--Si eutectic alloy solder. Pins 69 and 74 extend through the package 67 with insulating layers 73 surrounding the pins 69 and 74 to thereby electrically insulate the pins 69 and 74 from the electrically conductive layer 71 of the package 67. The gate electrode 75 and the emitters 62 are wire-bonded to the pins 69 and 74 through electrically conductive wires 68 and 70, so that an external voltage can be applied to the gate electrode 75 and the emitters 62. The bonding pad 66 constitutes a part of the tungsten silicide layer 53, and the wire 68 is directly bonded to the tungsten silicide layer 53. Since the emitters 62 are kept in electrical communication with the electrically conductive layer 71 through the adhesive layer 72, it is possible to apply a voltage to the emitters 62 through both the pin 74 and a lower surface of the field emission electron source at which the field emission electron source is secured to the package 67, by electrically connecting the pin 74 electrically insulated from the package 67 with the electrically conductive layer 71 through the wire 70.
Wire-bonding has been widely used for mounting of a semiconductor element because of its mass production capability. In wire-bonding, bonding strength is an important factor for acquiring reliability for products. In general, when wire-bonded to an aluminum bonding pad formed on a surface of a semiconductor device on which a circuit is to be formed, a wire made of gold or aluminum is bonded to the bonding pad by means of supersonic waves or both supersonic waves and heating.
As explained earlier with reference to FIGS. 1A to 1F, since the conventional method of fabricating a field emission electron source has the step of removing the aluminum or alumina sacrifice layer by soaking into phosphoric acid solution, it is not allowed to make a bonding pad of aluminum. Instead, a bonding pad in the above mentioned conventional field emission electron source is made of tungsten silicide which is resistive to phosphoric acid solution. When wire-bonded to a bonding pad made of tungsten silicide, a wire made of gold is unable to be used because of poor bonding force between tungsten silicide and gold. If a wire made of aluminum is to be used in place of a gold wire, it is impossible to obtain sufficient bonding force between tungsten silicide and aluminum. Specifically, there is merely obtained bonding strength ranging from about 3 grams to 7 grams at maximum.
In the above mentioned conventional field emission electron source, a wire is bonded to a bonding pad made of electrical conductor other than aluminum, such as tungsten silicide. Thus, if a wire made of gold is to be used for wire-bonding, there can be obtained only small bonding force, resulting in that wire-bonding cannot be sufficiently achieved. If a wire made of aluminum is to be used for wire-bonding, sufficient bonding force cannot be obtained, as mentioned earlier. As a result, products can have poor reliability.
Japanese Unexamined Patent Publication No. 3-280458 published on Dec. 11, 1991 has suggested a method of securing an external lead terminal to a metalized metal layer formed on a substrate through solder including Au and In ranging from 0.1 to 15.0 wt % based on Au.
Japanese Unexamined Patent Publication No. 3-250655 published on Nov. 8, 1991 has suggested a method of securing an external lead terminal to a metalized metal layer formed on a substrate through solder including Au, In ranging from 0.1 to 15.0 wt % based on the weight of Au, and at least one of Pd, Rh, Co and Cr ranging from 0.1 to 5.0 wt % based on the weight of Au. Both of Japanese Unexamined Patent Publications Nos. 3-280458 and 3-250655 are hereby incorporated by reference to extent that they are consistent herewith.
However, the methods suggested in those Publications cannot provide sufficient bonding force, if a wire made of gold or aluminum is to be used for wire-bonding.